Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A cellular wireless network may include a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices (whether or not technically operated by a human user), can operate. In turn, each base station may be coupled with network infrastructure, including one or more gateways and switches, that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or a packet-switched network such as the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs.
In practice, physical base station equipment in such a system may be configured to provide multiple coverage areas, differentiated from each other by direction, carrier frequency, or the like. Each coverage area may provide service on one or more carriers, and each carrier in each coverage area may define a cell. For example, if a base station provides three coverage areas and provides service on two carriers per coverage area, the base station provides six cells. In some examples, there can be multiple cells at the same physical location, each provided by the same base station, and each being on a different carrier.
In general, a cellular wireless network may operate in accordance with a particular radio access technology or “air interface protocol,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Communications on a cell can be frequency division duplex (FDD), in which the downlink and uplink operate on separate frequency channels, or time division duplex (TDD), in which the downlink and uplink operate on a shared frequency channel and are distinguished from each other over time. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) or Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover of UEs between cells, and functions related to air interface communication.
In accordance with the air interface protocol, air interface resources are mapped in the time domain and/or the frequency domain to provide discrete locations on the air interface for carrying communications. In a representative OFDMA network, for instance, the downlink of each cell is mapped over frequency and time into an array of resource elements, which define the locations on the air interface at which the base station can transmit data to UEs. In particular, the downlink is divided over frequency into a range of closely-spaced orthogonal subcarriers and is divided over time into a continuum of symbol time periods, thereby defining an array of resource elements each centered on a respective subcarrier and spanning a respective symbol time period. With this arrangement, as the base station has data to transmit to UEs, the base station may transmit the data in particular resource elements to the UE (i.e., at particular locations on the air interface).
By way of example, in accordance with the LTE protocol, the downlink of each cell spans a frequency bandwidth such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, and that frequency bandwidth is divided into 15 kHz subcarriers (i.e., subcarriers spaced apart from each other by 15 kHz). Further, the air interface is divided into a continuum of 10-millisecond (ms) frames, each frame is divided into ten 1 ms sub-frames, and each sub-frame is divided into two 0.5 ms slots. Each 1 ms downlink sub-frame is then further divided into 14 symbol time periods, each spanning 66.7 microseconds plus an added 4.69 microsecond guard band (cyclic prefix). With this arrangement, each sub-frame thus defines an array of resource elements, each centered on a 15 kHz subcarrier and spanning a symbol time period, and each such resource element may effectively carry a single orthogonal frequency division multiplexing (OFDM) symbol representing communication data.
In each frame, certain resource elements on the downlink are reserved for carrying particular types of data. For instance, resource elements at particular locations on the air interface are reserved for carrying synchronization signals, which enable UEs to synchronize their timing with that of the cell so that the UE can then read system information and evaluate the cell's coverage (e.g., a signal strength). Under LTE, for example, the base station provides primary and secondary synchronization signals on each cell. In particular, within a certain range of resource elements above and below the center frequency of the cell, the base station transmits a primary synchronization signal (PSS) in the last symbol period of the first slot (i.e., symbol period 6) within the first and sixth sub-frames (i.e., sub-frame 0 and sub-frame 5) of every frame. Further, the base station also transmits a secondary synchronization signal (SSS) in the second to last symbol period (i.e., symbol period 5) of those same slots and sub-frames, in the same range of range of resource elements above and below the center frequency of the cell. The PSS carries a coded value, and the SSS carries a coded value.
When a UE enters into coverage of a cell, the UE may engage in an initial access procedure to facilitate the UE communicating with the cell. In LTE, the UE first searches for the PSS of the cell (e.g., by looking for a PSS value that periodically recurs as expected) so as to lock onto the sub-frame timing of the cell. The UE will then read the SSS of the cell (e.g., by looking at the resource elements located one symbol period before the identified resource elements of the PSS) to lock onto the frame timing of the cell. Once the UE ascertains the PSS value and SSS value, the UE can then compute a physical cell identifier (PCI) of the cell (e.g., as a predefined function of those values). And given the PCI, the UE is then able to determine which resource elements on the air interface carry a reference signal, so that the UE can read and evaluate the strength of the reference signal. Based on the reference signal strength, the UE may select the cell and engage in a process of attaching to the cell so that a base station may then serve the UE on that cell. This procedure for synchronizing with and selecting a cell may also be generally referred to as a cell selection process.
The UE may use the above-described cell selection process in various scenarios. For example, when a UE powers on, a UE may use the cell selection process to select an initial cell on which the UE can attach and thereby gain access to the network. Additionally, for example, the UE may use the cell selection process after the UE has attached to an initial cell. For instance, when a UE is served by a particular cell, the UE may also regularly scan for other cells and evaluate the reference signal strength of those other cells in an effort to ensure that the UE operates in the best (e.g., strongest) coverage. Accordingly, when the UE detects other cells, the UE may engage in the above process so as to synchronize with each cell and evaluate each cell's respective reference signal strength. If the UE detects threshold weak reference signal strength from its serving cell and sufficiently strong reference signal strength from another cell, the UE may then engage in a handover process by which the UE transitions to be served by the other cell. In the idle mode, the UE may do this autonomously. Whereas, in the connected/active mode, the UE may report signal strengths to its serving cell when certain thresholds are met, and the cell may work to hand the UE over to another cell.
Under certain air interface protocols, a base station may be able to serve a UE concurrently on multiple cells (i.e., multiple carriers), to help increase the effective bandwidth and associated throughput available to the UE. This is known as carrier aggregation. By way of example, according to the LTE air interface protocol, the maximum bandwidth for a data transmission between a base station and a UE using a single carrier frequency is 20 MHz. By engaging in carrier aggregation, the base station may increase the number of resource elements provided to a UE by aggregating up to five carrier frequencies, and consequently increasing the maximum bandwidth to up to 100 MHz. To facilitate carrier aggregation service, the base station may designate one carrier as a primary carrier or primary cell (PCell) and the base station may designate each other carrier as a secondary carrier or secondary cell (SCell).
Depending on the desired implementation, a base station may be carrier aggregation capable or not. If a base station is carrier aggregation capable, the base station may have certain policies specifying which of its cells can be combined together to provide carrier aggregation. By way of example, a base station may have a carrier-aggregation policy for each cell that indicates whether the cell can be used as a PCell in combination with one or more other cell(s) as SCell(s). For instance, in one scenario, carrier-aggregation policies of a base station may indicate that a first cell and a second cell may be used as PCells, but a third cell may not be used as a PCell. Further, the carrier-aggregation policies may indicate that (i) the first cell can be a PCell with the second cell as an SCell, but not with the third cell as an SCell, whereas (ii) the second cell can be a PCell with both the first cell and the third cell as SCells. Thus, if the first cell is used as a PCell, the base station may aggregate two carriers, but if the second cell is used as a PCell, the base station may aggregate three carriers.
In practice, a base station may implement carrier-aggregation policies for a number of reasons. As one example, certain pairs of cells may be undesirable to combine because concurrent transmission on the carrier frequencies of the two cells could give rise to intermodulation distortion. For instance, concurrent transmission on two particular cells may combine to produce an undesirable radio frequency (RF) byproduct. If a base station engages in carrier aggregation with a UE on the two cells, the UE or base station may receive the RF byproduct, thus interfering with the transmissions on the two cells. As another example, a base station may implement carrier-aggregation policies that are dynamically modified over time based on the level of congestion in the control channel region of a particular cell. For instance, if all control signaling is configured to occur on PCells rather than SCells, and a control channel region of the particular cell is threshold highly congested, then it may be desirable to avoid using that cell as a PCell, but allow the cell to be used as an SCell.
UEs also may have different carrier aggregation capabilities. For example, some UEs may not support carrier aggregation, others may support aggregating up to two carriers, others may support aggregating up to three carriers, others may support aggregating up to four carriers, and still others may support aggregating up to five carriers. Additionally, for example, some UEs may support carrier aggregation for only TDD carriers, other UEs may support carrier aggregation for only FDD carriers, and still other UEs may support carrier aggregation for both TDD and FDD carriers. In further examples, the carrier aggregation capabilities of UEs may differ in still other ways.